1. Field of the Invention
The present invention relates to an imaging apparatus, more particularly, to an imaging apparatus capable of generating an appropriate color image even when a subject is imaged under a high contrast (dynamic range) condition.
2. Description of the Related Art
Recently, an interest in traffic safety and prevention of traffic accidents has been increasing, and as a technology in a drive assist system of a vehicle, research and development of on-vehicle monitoring imaging apparatuses have been carried out. Among those, there has been a problem on how to ensure monitoring performance of an imaging apparatus under a very high contrast condition due to, for example, back light in the evening and thus, under a condition of possible low visibility. More specifically, there has been a problem on how to ensure color reproducibility in the imaging apparatus in consistency with human's color vision characteristic in order to distinguish color of traffic lights and lines indicating lanes even under the condition of possible low visibility. Very high contrast of such a scene is over limitation of a dynamic range of a regular image pickup device. This causes overexposure and loss of gradation information and therefore halation where an image is in solid white occurs. This also causes, to the contrary, underexposure and loss of gradation information and therefore blacking-out where an image is in solid black occurs.
Thus, an image pickup device has been proposed, which is capable of expanding the dynamic range by introducing non-linear characteristic into a relationship between input brightness and output signals.
In such an image pickup device having non-linear characteristic, a method has been proposed, where output signals of an area having non-linear characteristic are converted into linear signals (linearization); then various color signal processings are performed; and afterwards a width in bits is narrowed according to a monitor to which an image is output (see, for example, Japanese patent application publication No. 2001-86402).
Furthermore, an image pickup device has been proposed, where, without linearizing output signals of the image pickup device, specific white balance correction with LUT (Look Up Table) is performed such that photoelectric conversion characteristics in R, G, and B which are different from each other are matched with any of photoelectric conversion characteristics in R, G, and B to improve efficiency (simplifying, speeding-up) in following gradation conversion processing or color signal processing (see, for example, Japanese patent application publication No. 2006-270622).
However, an image processing apparatus disclosed in Japanese patent application publication No. 2001-86402, signals are compressed by narrowing a width in bits during signal processing steps. This means that information on low brightness parts is reduced and therefore color images taking full advantage of the original dynamic range of the image pickup device cannot be obtained. Accordingly, for example, in a scene where a subject having low brightness and a subject having high brightness are concurrently present, if output signals obtained from an image area of the low brightness subject are included in lower bits to be reduced, color of the low brightness subject cannot be reproduced in a final output image. That is, there is a problem in that an appropriate color image cannot be generated if the subject is imaged under a condition of high contrast (dynamic range) situation.
Furthermore, in an imaging apparatus disclosed in Japanese patent application publication No. 2006-270622, after performing gradation conversion, color signal processing which is the same as one performed on signals having linear characteristic. However, if the color signal processing (linear interpolating processing, linear matrix processing, and the like), which is the same as one performed on signals having linear characteristic, is applied to signals having non-linear characteristic, color deviation may occur and therefore there is a problem in that precise color reproducibility cannot be achieved.